Satellite earth receiving stations, or television receive only (TVRO) stations, have become enormously popular among the television consuming public in the past four to five years. Individuals, particularly those whose regular television reception is poor and for whom cable T.V. may not be available, have been installing such earth stations as the origination point for all of the iput signals to the television set. Earth stations are constructed so as to minimize interference from terrestrial microwave transmissions, some of which share the same 4-GHZ spectrum, and the receiving antennas must be situated so as to allow an unobstructed line of sight path to each of the desired satellites.
The largest component of the earth station is the receiving antenna which, in the case of home use, is a parabolic reflector or dish which may be from 4 feet to about 12 feet in diameter. The most popular dishes in the home market are from 6 feet to 12 feet in diameter. A waveguide or feedhorn is used to collect the signals at the focal point of the parabolic reflecting surface and to transmit them to a low noise pre-amplifier. Electronic processing, not a part of the present invention, is conventionally used to convert the signals to the different television channels.
In parabolic antennas from 6 feet to about 12 feet in diameter, the feedhorn is most often fixedly mounted at the end of a solitary support rod or arm, sometimes referred to as a buttonhook. The buttonhook extends axially outwardly from the concave side of the dish at its vertex for a predetermined distance such that the feedhorn it carries is situated precisely at the focal point of the reflector.
Where a buttonhook or similar type of support arm is used for the feedhorn, it is generally desirable to provide additional structural support for the feedhorn through the use of tensioned stays or guy wires. Guy wires are utilized to prevent the feedhorn from being swayed laterally or otherwise moved away from the focal point of the dish during periods of loads or stress on the dish, such as might be caused by wind, ice, snow or the like which have a tendency to deform, vibrate or otherwise adversely affect the stability of the structure. Any swaying or other movement induced in the buttonhook due to such stress can affect picture quality adversely. This is particularly true at the higher frequencies in the usual bands of operation.
Heretofore, structural support for the feedhorn has been provided through the use of a plurality of stays or guy wires which typically run from the feed itself to a point of termination at the peripheral edge of the parabolic dish. In a typical installation, there would be four such guy wires spaced apart by approximately 90.degree..
This prior arrangement has given rise to a number of problems and disadvantages. For example, many TVRO parabolic dishes are mounted at such a height that the upper most guy wire is often more than 10 feet above the ground. It is therefore difficult for an individual to access the connection point at the edge of the dish in order to adjust the tension in the guy wire. Even more significantly, since the guy wires must be taut in order to be effective, there is a tendency for such wires to induce a moment of force in the dish itself which causes the dish to deform inwardly toward the feedhorn. Any such deformation in the surface of the parabolic dish tends to alter the location of the focal point of the dish with devastating effects on picture quality. An additional disadvantage of the prior type of guy wire system is that the guy wires attached at the edge of the parabolic reflector tend to add structural support only to resist wind or other loads directed against the front or concave side of the reflector. Stress loads incident on the back of a dish having the prior type of guy wire system have a tendency to cause the wires to relax their tension thereby rendering them ineffective.
Other mounting techniques have been utilized heretofore in an effort to stabilize the position of the signal receiving feedhorn. For example, one prior type of arrangement includes the use of a tripod formed of rigid aluminum rods which are fixed to the feedhorn and bolted directly to the surface of the parabolic reflector. Such a tripod arrangement does not necessarily avoid deformation of the dish and it is not susceptible of adjustment after installation.